CLASSIFICATION OF ACARINA. 29 



divisions are wholly artificial, although convenient and 

 even necessary, but the important matter is to keep the 

 groups of the same degree of fairly equal value in fact 

 as well as in name, and to have a classification which 

 gives as correct an idea as possible of the relationships 

 or resemblances which we actually find in nature. 

 Raising the Acarina into a class would, it seems to me, 

 separate them from such closely allied creatures as the 

 Phalangidas in a way which is not desirable. Another 

 point Avhich does not strike me as quite happy is the 

 establishment of the order Hydracarina. The only 

 bond which unites the families of which this group is 

 composed to one another, and separates them from the 

 Prostigmata, is the aquatic habitat, and this does not 

 seem sufficient to found an order upon. The Hydrach- 

 nidge appear to me by their anatomy, and everything 

 except their habitat, to be as or more closely allied to 

 the Trombididae as to the Halicaridse, and to be nearer 

 to the Trombididse than the Tarsonemidse are. The 

 close relationship between the internal anatomy of the 

 Hydrachnidas and the Trombididae was long since 

 proved by Croneberg.* 



My views as to the division of the Oribatidse (Cane- 

 strini's Cryptostigmata) into sub-families (Families 

 Canestrini) I have lately set out elsewhere. t 



In 1891 Dr. Trouessart, of Paris, published a classi- 

 fication of the Acarina, J which, although I cannot 

 entirely agree with every part of it, is, I am inclined to 

 think, on the whole the best hitherto published. The 

 following is a translation of the explanatory table of 

 the paper : 



* " On the Anatomy of Eylais extendens, Miiller, with Observations 

 on Allied Forms; " in Russian, in ' Nachr. ges. d. Freunde d. Natur- 

 kunde/ Moscow, 1878. A short summary in German called " Ueber 

 den Bau der Hydrachniden," in Zool. Anz.,' 1878, No. 14, p. 316; 

 " Ueber den bau von Trombidium," in ' Bull. Soc. Moscou,' 1879, pt. 2, 

 p. 234. 



f " Oribatidse," in ' Das Tierreich,' Lief. 3, 1898, Berlin. 



" Considerations Generales sur la classification des Acariens 

 suivies d'un essai de classification nouvelle," ' Rev. Sci. Nat. Ouest.,' 

 1891, pp. 289-308 (1892), pp. 2055. 



